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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I'm trying to implement a thermal laser power meter using the classical thermistor-power resistor device.
It consist in two 47ohm 1W power resistors epoxied to a black alluminium square with a thermistor. THe resistors are heated to a specific temperature (measured by the termistor) till equilibrium. When the laser shines the black face it will cause heating, and the feedback circuit will try to mantain the same themperature reducing the load on the heating resistors. The difference in loads equals the laser power.
Speficifically i'm using a 10K thermistor, an LM358 opamp, 5V :
The non-inverting input has a constant adjustable voltage (it is the "temperature" setpoint) THe noninverting input has the voltate coming from the voltage divider formed by the thermistor (upper arm) and a 10K resistor (lower arm). The setpoint voltage is 2.96 V. The opamp regulates the load the inverting input voltage reaches 2.96 V and then stops. The problem is that the opamp, instead of regulating it linearly reducing the load to a speficic (background) value completely cuts off power, switching it back on when the temperature starts to drop. In fact the high DC openloop gain of the opamp makes it work as a comparator. So i have to find a way to reduce the gain, say to 15-20dB. Would be it enough of have I to think about a fancy compensation network?
Any suggestions?
EDIT:
I added a 47Kohm feedback resistor between inverting input and output and now the loop is stable. The sensitivity is monstrous, it can detect the air current generated by me moving accross the room, my hand close to it and so on, so i added a shield (a half ping pong ball painted in black with a hole). I tried to measure the power of a 3.5mW aixys module obtaining these values: (current measured using 1 ohm resistor)
Without laser shining on it: 92,6 mA @ 2.25 V heating power (199.77 mW) (20 seconds after stabilization) With laser shining on it: 91,7 mA @ 2.23 V heating power (196.08 mW) (as above)
Difference: 3.7 mW, that's quite reasonable
To measure lasers above 200mW i need to increase my heating power or temperature setpoint.
THe best would be to add a PIC16F88 to control automatically zero and perform these calculations using the on-board ADCs, adjust pre-heat delays and so on.
Of course it is a dirty/junky weekend project, with a little more careful designing/building and themal insulation i can probably measure down to a 1mW without problems.
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Building thermal laser power meter
